Recently the partial-response maximum-likelihood principle which was already used in communication systems has found application also in magnetic recording systems. It allows to increase the recording density on the medium, or, on the other hand, to lower the error rate during the readback process for otherwise equal recording channels.
PRML (partial-response maximum-likelihood) magnetic recording applications have been described e.g. in European patent application No. 0,133,480 entitled "Method and Apparatus for Decoding the Output Signal of a Partial-Response Communication or Recording-Device Channel", and in an article by R. W. Wood et al. "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel", published in IEEE Transactions on Communications, Vol. COM-34, No. 5, May 1986, pp. 454-461. No details on gain control in the readback apparatus of a PRML magnetic recording system are disclosed in these publications.
An important task of the magnetic readback apparatus is the adaptation of the varying amplitude levels of the readback signal received from the magnetic reading head to the nominal amplitude levels as required by the detector.
In U.S. Pat. No. 4,346,411 entitled "Amplitude-Sensitive Three-Level Detector for Derivative Read Back Channel of Magnetic Storage Device", an apparatus is disclosed for readback of magnetic recording signals which includes a variable-gain amplifier. However, this apparatus is not suited for PRML systems.
The charge-pump principle has been used since several years in phase-locked loop circuitry for converting discrete signals into analog quantities for controlling the oscillators, as is e.g. described in the article "Charge-Pump Phase-Lock Loops" by F. M. Gardner, IEEE Transactions on Communications, Vol. COM-28, No. 11, November 1980, pp. 1849-1858. The charge-pump principle can also be used for deriving the control voltage for a variable gain amplifier from a digital version of the amplified signal in a gain control loop.
One problem in the readback of recorded signals are the short-time signal amplitude drops which are due to small defects in the magnetic recording medium. Such signal amplitude drops which may have a duration of several tens of symbol intervals will result in false detection of data.
These short-duration signal drops cannot be satisfactorily compensated by presently known gain control loops. They represent the limiting factor in the error rate performance and in the stability of the timing and gain control loops of the detection circuitry in the readback system.